This invention relates to multichannel superheterodyne radio receivers using frequency synthesizing apparatus to develop the required local oscillator signal and having a programmable memory for controlling the frequency to which the receiver is tuned when it is operative on any given channel and, in particular, it relates to apparatus for programming data into such a memory. The invention is particularly suited to the provision of such radio receivers for receiving signals on the frequencies designated by the United States Federal Communications Commission as being available for assignment to the Public Safety Radio Services.
Radio receivers using frequency synthesizing apparatus to generate the required local oscillator signals are well known. Moreover, such receivers using electrically programmable memories to control the operation of the frequency synthesizing apparatus are also known. See, for example, the discussion in "RCA Solid State Databook Series: COS/MOS Digital Integrated Circuits", No. SSD-203A, published by RCA Corporation, Box 3200, Somerville, New Jersey, particularly at pages 419-433. Further, scanning radio receivers which are capable of sequentially tuning to a plurality of individual, spaced apart radio frequencies are well known. Certain difficulties have arisen in the design of radio receivers which combine these features.
One example where such difficulties have arisen is in the design of receivers for reception of signals on the radio frequencies designated by the United States Federal Communications Commission as being assigned to the Public Safety Radio Service. Such radio receivers previously known which are capable of automatically and sequentially tuning to a plurality of those frequencies have most often included a crystal controlled oscillator with individual crystals for generating the local oscillator signals required for reception of signals at each of the tuned frequencies. Examples of such receivers are the multichannel scanning receiver disclosed in the copending application of G. H. Fathauer filed on Nov. 4, 1974, Ser. No. 520,438, now U.S. Pat. No. 3,987,400 and the receivers discussed in the patents and application cited therein. Receivers such as those have had the advantages of extreme ease of tuning and stability of operation; however, they have had the disadvantage that they required one crystal for each frequency to be received. The cost of crystals can become a significant amount where a receiver is to be capable of receiving signals at a large number of different frequencies. Moreover, when it is desired to alter one of the received frequencies corresponding to one of the channels, the crystal must be changed, resulting in further expense and inconvenience to the user. Receivers using frequency synthesizers to generate the local oscillator signals provide ease of tuning and stability of operation similar to that of crystal receivers, but have the advantage that separate crystals are not required for each frequency to be received. Moreover, in such receivers which include electrically programmable memories to control the frequency to which the receiver is tuned when it is operative on any given channel, when it is desired to change the frequency corresponding to a channel this can be done without the requirement of substituting any new components into the receiver.
However, the circuitry suggested by the prior art to provide a multichannel, scanning radio receiver using frequency synthesizing circuitry would fail to fulfill one or more of a plurality of desiderata. First, it is desirable that the memory used in any such receiver be nonvolatile and capable of maintaining the information stored therein over long periods of time, preferably without the use of any external power source such as a battery. It is desirable that the circuitry provided to program the memory be one which is easy for a relatively unskilled person to operate. It is desirable that the memory programming circuitry not overawe prospective users thereof; this may be achieved by designing the memory programming circuitry such that the user operated controls have a function and appearance similar to the controls of scanning radio receivers of the prior art to which users are already accustomed. It is desirable that the memory programming circuitry include provisions for preventing accidental erasure of the information stored in the receiver either by inadvertant manipulation of the receiver's controls or by electrical transient conditions which might arise. The memory programming circuit should also be compatible with the remainder of the receiver circuits so that features found in receivers of the prior art may be maintained. Finally, the memory programming circuitry should add as little complexity and expense to the receiver as possible.